Process for dissolving mandrel wire of a filament coil

ABSTRACT

Gaseous nitrogen oxides generated at the time of dissolution of the mandrel wire of a filament coil can be accurately removed by adding an inhibitor for generation of gaseous nitrogen oxides to a resolving solution comprising nitric acid, water and at least one member selected from sulfuric, hydrochloric and phosphoric acids for dissolving the mandrel wire. Examples of said inhibitor are an oxidizing agent composed of an inorganic peroxide such as permanganic acid salt or perchloric acid salt and inorganic or organic reducing agents.

United States Patent [191 Hinonishi et al.

[ Apr. 30, 1 974 PROCESS FOR DISSOLVING MANDREL WIRE OF A F ILAMENT COIL [75] Inventors: Yoshiteru Hinonishi; Jyusaku Machida; ToheiTakayama, all of Tokyo, Japan [30] Foreign Application Priority Data July 14, 1971 Japan 46-51712 [52] US. Cl 29/25.l8, 29/423, 423/393 [51] Int. Cl. HOlj 9/14, B23p 17/00 [58] Field of Search 29/423, 25.11, 25.17, 25.18; 72/66; 313/12, 344; 423/393, 395; 140/715, 71.6

[56] References Cited 7 UNITED STATES PATENTS 3,110,098 11/1963 Sobierajski 29/423 5/1968 Waldorf ..23/1s7 1 1937 Zabel ..29/25.1sx

Primary Examiner-Roy Lake Assistant Examiner.l. W. Davie Attorney, Agent, or Firm-Craig and Antonelli [57] ABSTRACT Gaseous nitrogen oxides generated at the time of dissolution of the mandrel wire of a filament coil can be accurately removed by adding an inhibitor for generation of gaseous nitrogen oxides to a resolving solution comprising nitric acid, water and at least one member selected from sulfuric, hydrochloric and phosphoric acids for dissolving the mandrel wire. Examples of said inhibitor are an oxidizing agent composed of an inorganic peroxide such as permanganic acid salt or perchloric acid salt and inorganic or organic reducing agents.

13 Claims, No Drawings PROCESS FOR DISSOLVING MANDREL WIRE OF A FILAMENT COIL BACKGROUND OF THE INVENTION Generally, a filament coil used as the electrode of such light source as incandescent lamp or fluorescent discharge lamp or such electron tube as vacuum tube is produced by winding a fine tungsten wire around a mandrel wire to form a coil and, if necessary, further winding the thus formed coil around other mandrel wire to form a desired coil, e.g. a single or multiple coil. However, the mandrel wire, which has been used to form the tungsten wire into a desired filament coil, is required to be removed after formation of the filament coil. For the removal of said mandrel wire, there has been adopted a process in which only the mandrel wire is chemically dissolved. On the other hand, in forming a tungsten wire into a filament coil by winding, a strain is produced inside the tungsten wire due to the winding to deform the resulting coil or disorder the coil pitches. When the thus obtained filament coil is used as an electrode after dissolving the mandrel wire, the filament coil is greatly deteriorated in efficiency to become unusable. In order to remove the said internal strain, the tungsten wire after formation of coil should be treated at a high temperature. However, if a part of the metal constituting the mandrel wire diffuses to the tungsten wire at the time of high temperature treatment, the tungsten wire becomes brittle and is deformed or cut during the use thereof as the electrode of a light source or electron tube to become useless as the electrode and to greatly shorten the life of the light source or the like. Accordingly, the mandrel wire is required to satisfy such conditions that it can withstand the heat treatment temperature for removal of internal strain of the tungsten wire and can be easily dissolved after formation of filament coil. As metals satisfying the above conditions, there are molybdenum and nickel silver (an alloy comprising 45 to 65 percent of Cu, 6 to 35 percent of Ni, to 35 percent of Zn and slight amounts of Mn, Fe and Pb; m.p. 1,050C. or more). In order to make the resulting filament coil favorable in efficiency, the'heat treatment of the tungsten wire after formation of coil is preferably effected at a temperature as high as possible. In case the heat treatment is carried out at about 1,400C. for example, a molybdenum wire having a melting point of 2,610C. is used as the mandrel wire. However, if the internal strain of filament coil can be removed even when the heat treatment temperature is about 800C, and if the resulting coil is not particularly required to be high in efficiency as a filament coil, the heat treatment is conducted at 800C. or above. The nickel silver is suitable as a mandrel wire for formation of such filament coil.

A filament coil is produced by winding a tungsten wire around a mandrel wire to form a coil, heat-treating the coil to remove the internal strain of the tungsten wire and then removing the mandrel wire from the interior of the coil. I-Ieretofore, the mandrel wire has been removed by dissolving said wire with a resolving solution comprising nitric acid, sulfuric acid and water. That is, in case a molybdenum wire is used as the mandrel wire, there has been adopted, for example, such a process that a filament coil having the molybdenum mandrel wire inside the coil is charged into a resolving solution comprising 3.75 parts by weight of 98 percent nitric acid, 2.7 parts by weight of 98 percent sulfuric acid and 1 part by weight of water, and then the solution is heated to dissolve the molybdenum mandrel wire. In the above manner, only the molybdenum mandrel wire is dissolved and the tungsten filament coil is left in the resolving solution, thereby completing the dissolving operation. However, according to the dissolving process using the aforesaid resolving solution comprising nitric acid, sulfuric acid and water, the nitric acid in the resolving solution is gradually converted to nitrous acid with dissolution of the mandrel wire, and said nitrous acid generates large amounts of various gaseous nitrogen oxides such as, for example, NO and N0 These gaseous nitrogen oxides are harmful and give injuries to the human body and living things when liberated into the air, so that it is extremely important from the standpoint of environment sanitation to not liberate such air-polluting substances into the air. Heretofore, the gaseous nitrogen oxides generated at the time of dissolution of mandrel wire have been removed by washing air containing said gaseous nitrogen oxides with water or alkali solution, but it has been impossible to completely remove said oxides because the washing power is insufficient and long treatment time and enormous installations are required. On the other hand, the dissolving operation of the mandrel wire inside the aforesaid filament coil is carried out in a room, so that it is necessary for sanitary reasons to not allow the gaseous nitrogen oxides to stay in the room. Accordingly, the gaseous nitrogen oxides are discharged out of the room by use of a strong suction means. When discharged out of the room, however, the said oxides are diluted with air, so that the washing of the discharged oxides results in the washing of the diluted oxides. For the above reason, the complete treatment of said oxides is extremely difficult, and untreated gaseous nitrogen oxides are liberated into the air to bring about a cause for air pollution. As mentioned above, the washing treatment using water or alkali solution is less in washing effect. In order to make the amounts of gaseous nitrogen oxides so slight as to be negligible as airpolluting substances even when liberated into the air, enourmous treatment equipments, places and expenses are required to give great influence to the costs of products, and the like. Accordingly, it has been difficult'to establish an installation for commercial scale treatment of said gaseous nitrogen oxides.

SUMMARY OF THE INVENTION The present invention relates to a process for accurately removing, overcoming the drawbacks of the prior art process and by a simple procedure, gaseous nitrogen oxides generated at the time of dissolution of the mandrel wire of a filament coil, in which process an inhibitor for generation of gaseous nitrogen oxides is added to a resolving solution for dissolving the mandrel wire to inhibit the formation of nitrous acid, which becomes a source for generation of gaseous nitrogen oxides at the time of dissolution of the mandrel wire, and nitrous acid formed is converted by means of said inhibitor into nitric acid or decomposed into harmless substances such as nitrogen, carbon dioxide, water and the like to prevent the liberation of gaseous nitrogen oxides into the air.

An object of the present invention is to provide a process for dissolving the mandrel 'wire of a filament coil without generation of air-polluting substances, in which process an inhibitor for generation of gaseous nitrogen oxides is added to a resolving solution for dissolving the mandrel wire to simply prevent the generation of gaseous nitrogen oxides.

Another object of the invention is to provide a process for dissolving the mandrel wire of a filament coil in which process gaseous nitrogen oxides which become causes for air pollution are completely inhibited from generation and are prevented from being liberated into the air.

A further object of the invention is to provide a process for dissolving the mandrel wire of a filament coil, in which process an inhibitor for generation of gaseous nitrogen oxides is added to an aqueous solution containing nitric acid to form a resolving solution for dissolving the mandrel wire, so that in dissolving the mandrel wire with said resolving solution, nitrous acid derived from the nitric acid is inhibited by means of said inhibitor from being converted into gaseous nitrogen oxides and is converted into other harmless substances, thereby preventing the generation of harmful gases.

A still further object of the invention is to provide a process for dissolving the mandrel wire of a filament coil, in which process the dissolution of tungsten wire constituting the filament coil is prevented to obtain a filament coil less in weight loss and stable in efficiency.

DETAILED EXPLANATION OF THE INVENTION This invention relates to a process for dissolving the mandrel wire of a filament coil by means of a resolving solution comprising nitric acid, water and at least one member selected from sulfuric, hydrochloric acid, phosphoric acids, characterized in that the resolving solution is incorporated with an inhibitor for generation of gaseous nitrogen oxides, and nitrous acid formed in the dissolving step which becomes a cause for generation of gaseous nitrogen oxides is converted by means of said inhibitor into nitric acid or decomposed into harmless substances such as nitrogen, carbon dioxide, water and the like, so that no gaseous nitrogen oxides are formed and liberated into the air.

According to the present invention, the resolving solution is prepared by adding to the above-mentioned aqueous nitric acid solution or nitric acid-containing solution at least one oxidizing agent composed of an inorganic peroxide such as permanganic acid salt or potassium perchlorate, at least one inorganic reducing agent composed of an ammonium salt such as ammonium nitrate, ammonium chloride, ammonium sulfate or ammonium phosphate, at least one organic reducing agent composed of urea or oxalic acid, or a combination ofsaid inorganic reducing agent and organic reducing agent. Intothis resolving solution is charged a filament coil produced by winding a tungsten wire around a mandrel wire made of molybdenum or nickel silver to dissolve the mandrel wire with the resolving solution. At the time when the mandrel wire is dissolved, the resolving solution reacts with the mandrel wire, whereby the nitric acid becomes nitrous acid. According to the prior art, the thus formed nitrous acid is decomposed to form gaseous nitrogen oxides (NO, N N0 According to the present invention, however, the nitrous acid is converted into nitric acid by means of the aforesaid oxidizing agent in the resolving solution, or is decomposed into water and an entirely harmless gas such as nitrogen or a mixture of nitrogen and carbon dioxide by means of the aforesaid inorganic or organic reducing agent in the resolving solution, so that the nitrous acid is prevented from being liberated as gaseous nitrogen oxides into the air. Reaction formulas of the above-mentioned oxidizing agents or reducing agents as inhibitors for generation of gaseous nitrogen oxides have not theoretically been clarified yet, but it is inferred that the generation of gaseous nitrogen oxides can be prevented by the combination of various reaction formulas.

The present invention is illustrated in detail below with reference to examples.

EXAMPLE 1 To an aqueous solution comprising 29 g. of 62 percent nitric acid and 87 g. of water was added 8 g. of potassium permanganate as an inhibitor for generation of gaseous nitrogen oxides to prepare a resolving solution mixture. Into this resolving solution mixture was charged a double coil filament prepared by winding a tungsten wire around a molybdenum mandrel wire (molybdenum weight 4 g.), whereby the molybdenum mandrel wire was completely dissolved and, nitrous acid formed in the resolving solution was converted into nitric acid by the oxidizing power of the potassium permanganate, and there was scarcely the case where the nitrous acid was decomposed to gaseous nitrogen oxides and liberated into the air. The double coil filament taken out of the resolving solution after dissolution of the mandrel wire had more or less amount of a deposit on the surface. When the deposit was removed by acid-washing, however, there was obtained a filament for incandescent lamp which was substantially identical in efficiency with that obtained by washing the double coil filament with a resolving solution comprising nitric acid, sulfuric acid and water.

On the other hand, the resolving solution after dissolution of the mandrel wire was analyzed to detect no tungsten. F rom this, it was confirmed that the resolving solution used in this example to dissolve the mandrel wire did not dissolve the tungsten wire even partly to bring about no decrease in weight of. the resulting filament coil and thus gave no detrimental effect to the filament coil.

EXAMPLE 2 The molybdenum mandrel wire of the same double coil filament as in Example 1 was dissolved by use of a resolving solution comprising 29 g. of 62 percent nitric acid, 2.5 g. of 98 percent sulfuric acid, 87 g. of water and 8 g. of potassium permanganate, whereby the dissolution was completed within about 15 minutes.

EXAMPLE 3 The molybdenum mandrel wire of the same double coil filament as in Example 1 was dissolved by use of a resolving solution comprising 29 g. of 62 percent nitric acid, 2.5 g. of 98 percent sulfuric acid, 87 g. of water, 8 g. of potassium permanganate and l g. of potassium perchlorate (commercially available product), whereby the dissolution was complete within about l0 minutes. r

In this example, the potassium perchlorate formed a mixed crystal with the potassium permanganate and converted the formed nitrous acid into nitric acid to display the action of preventing the generation of gaseous nitrogen oxide, like the potassium permanganate. At the same time, the potassium perchlorate was strong in oxidizing power, so that the time of dissolving the molybdenum mandrel wire could be shortened.

The results obtained in Examples 2 and 3 were similar to those in Example 1.

EXAMPLE 4 A resolving solution comprising 25 g. of 85 percent phosphoric acid, 29 g. of 62 percent nitric acid, 70 g. of water and g. of ammonium chloride was maintained at below 80C., and a double coil filament having a molybdenum mandrel wire of 3 g. in weight was charged in said solution to dissolve the mandrel wire, whereby the dissolution was complete within about 20 minutes. At the time of dissolution of the molybdenum mandrel wire, nitrous acid was formed, but the ammonium chloride acted as a decomposing agent to decompose the nitrous acid into nitrogen gas and water, and prevented said acid from converting into gaseous nitrogen oxides, so that the amounts of gaseous nitrogen oxides liberated into the air were quite slight.

On the other hand, the resolving solution after dissolution of the molybdenum mandrel wire was analyzed, whereby tungsten was detected to confirm that a part of the filament coil had been dissolved. However, the amount of dissolved filament coil was quite slight and weight loss of the filament coil was not such an extent as to give any detrimental effect to the characteristics of the filament coil when used as an electrode for light source.

EXAMPLE 5 A resolving solution comprising 25 g. of 85 percent phosphoric acid, 25 g. of 62 percent nitric acid, 65 g. of water and 3.5 g. of ammonium sulfate was maintained at below about 80C. and was used to dissolve the molybdenum mandrel wire of the double coil filament used in Example 4, whereby the dissolution of the mandrel wire was complete within about minutes.

EXAMPLE 6 A resolving solution comprising 10 g. of 85 percent phosphoric acid, 25 g. of 62 percent nitric acid, 70 g. of water, l0 g. of ammonium phosphate and 10 g. of ammonium nitrate was maintained at below about 90C. and was used to dissolve the molybdenum mandrel wire of the double coil filament used in Example 4, whereby the dissolution of the mandrel wire was complete within about 10 minutes.

EXAMPLE 7 A resolving solution comprising 15 g. of 62 percent nitric acid, g. of 98 percent sulfuric acid, 65 g. of water and 4 g. of ammonium sulfate was maintained below about 70C. and was used to dissolve the molybdenum mandrel wire of the double coil filament used in Example 4, whereby the dissolution was complete within about minutes.

EXAMPLE 8 A resolving solution comprising 20 g. of 62 percent nitric acid, 15 g. of hydrochloric acid, 65 g. of water and 5 g. of ammonium chloride was maintained below about 70C. and was used to dissolve the molybdenum mandrel wire of the double coil filament used in Example 4, whereby the dissolution was complete within about 25 minutes.

In each of the above-mentioned Examples 5 to 8, the nitrous acid formed at the time of dissolution of the molybdenum mandrel wire was not converted into gaseous nitrogen oxides but was reduced and decomposed by means of ammonium sulfate, ammonium phosphate, ammonium nitrate or ammonium chloride, so that the liberation of gaseous nitrogen oxides into the air was prevented and favorable results were obtained from the standpoint of weight loss of the filament coil, like in Example 4.

EXAMPLE 9 A resolving solution comprising 10 g. of 62 percent nitric acid, g. of water and 4 g. of urea was maintained at below about C. and was used to dissolve the molybdenum mandrel core (2 g.) of a double coil filament, whereby the dissolution of the mandrel wire was complete within about 20 minutes. Although nitrous acid, which is easily converted into gaseous nitrogen oxides, was formed at the time of dissolution of the mandrel wire, said acid was decomposed into nitrogen, carbon dioxide by the reducing action of the urea, and the amounts of gaseous nitrogen oxides liberated into the air were so slight as to bring about no cause for air pollution.

On the other hand, the resolving solution after dissolution of the molybdenum mandrel wire was analyzed, whereby tungsten was detected to find that a part of the tungsten wire constituting the filament coil had been dissolved. However, the amount of dissolved tungsten wire was not more than 0.5 percent of the theoretical amount and was not such an extent as to injure the efficiency of the filament coil.

EXAMPLE 10 An aqueous solution containing 86 g. of 85 percent phosphoric acid, 10 g. of 62 percent nitric acid and 3 g. of urea was maintained at below about 90C., and was used to dissolve the molybdenum mandrel wire (molybdenum weight 1.5 g.) of a double coil filament, whereby themandrel wire could be dissolved within about 15 minutes.

EXAMPLE 1 l A resolving solution comprising 67 g. of 85 percent phosphoric acid, 8 g. of 62 percent nitric acid, 25 g. of water and 2.5 g. of urea was maintained at below about C., and was used to dissolve the molybdenum man drel wire (2 g.) of a double coil filament, whereby the dissolution was complete within about 25 minutes.

In each of the above-mentioned Examples 10 and l l, the urea acted, like in Example 9, as a decomposing agent for nitrous acid, which becomes a cause for generation of gaseous nitrogen oxides, and decomposed said acid into harmless nitrogen and carbon dioxide, and there was scarcely the case where gaseous nitrogen oxides were liberated into the air. Moreover, the weight loss of the tungsten wire was quite slight. The double coil filament of Example ll was used as an electrode for fluorescent discharge lamp to find that the filament was not injured in efficiency as the electrode.

In all the above Examples, there were illustrated the cases where a molybdenum wire was used as the mandrel wire, around which a tungsten wire was to be wound. However, in the cases where a nickel silver wire was used as the mandrel wire, there were also obtained the effects that the generation of gaseous nitrogen oxides and the weight loss of filament coil could be prevented, like in the aforesaid Examples.

As mentioned above, the present invention is applicable to the cases where the mandrel wire of a filament coil composed of a tungsten wire is dissolved by means of a resolving solution containing nitric acid, and the mandrel wires usable in the present invention are not limited only to molybdenum and nickel silver wires but include other materials that can be used as mandrel wires.

What is claimed is:

1. In a process for dissolving the mandrel wire of a filament coil comprising the steps of l. preparing the filament coil by means of winding a tungsten wire around a mandrel wire,

2. dipping said filament coil into a resolving solution containing nitric acid to dissolve the mandrel wire, and

3. washing said filament coil, the improvement comprising adding an inhibitor to the resolving solution to prevent generation of gaseous nitrogen oxides at the time of dissolution of the mandrel wire, the inhibitor consisting essentially of an inorganic peroxide, an ammonium salt or an organic reducing agent.

2. The process according to claim 1, wherein said resolving solution additionally contains at least one member selected from the group consisting of sulfuric acid, hydrochloric acid and phosphoric acid.

3. The process according to claim 1, wherein the inhibitor is an inorganic peroxide.

4. The process according to claim 3, wherein the inorganic peroxide is at least one member selected from the group consisting of permanganic acid salt and perchloric acid salt.

5. The process according to claim 1, wherein the inhibitor is at least one member selected from the group consisting of ammonium nitrate, ammonium chloride, ammonium sulfate and ammonium phosphate.

6. The process according to claim 1, wherein the inhibitor is an organic reducing agent.

7. The process according to claim 6, wherein the organic reducing agent is at least one member selected from the group consisting of urea and oxylic acid.

8. The process according to claim 1, wherein said mandrel wire is made from molybdenum or nickel silver.

9. The process according to claim 1, wherein said inhibitor is selected from the group consisting of potassium permanganate, potassium perchlorate, ammonium chloride, ammonium sulfate, ammonium phosphate, ammonium nitrate, urea and mixtures thereof.

10. The process according to claim 9, wherein said inhibitor is a mixture of potassium permanganate and potassium perchlorate.

11. The process according to claim 1 wherein said inhibitor is present in said resolving solution in an amount sufficient to absorb substantially all the gaseous nitrogen oxides produced when said filament coil is dipped into said resolving solution.

12. The process according to claim 11, wherein said resolving solution contains at least about 3 percent inhibitor.

13. The process according to claim 1, wherein said mandrel wire is a nickel silver alloy comprising about 45 to 65 percent copper, 60 to 35 percent nickel and 15 to 35 percent zinc. 

2. The process according to claim 1, wherein said resolving solution additionally contains at least one member selected from the group consisting of sulfuric acid, hydrochloric acid and phosphoric acid.
 2. dipping said filament coil into a resolving solution containing nitric acid to dissolve the mandrel wire, and
 3. washing said filament coil, the improvement comprising adding an inhibitor to the resolving solution to prevent generation of gaseous nitrogen oxides at the time of dissolution of the mandrel wire, the inhibitor consisting essentially of an inorganic peroxide, an ammonium salt or an organic reducing agent.
 3. The process according to claim 1, wherein the inhibitor is an inorganic peroxide.
 4. The process according to claim 3, wherein the inorganic peroxide is at least one member selected from the group consisting of permanganic acid salt and perchloric acid salt.
 5. The process according to claim 1, wherein the inhibitor is at least one member selected from the group consisting of ammonium nitrate, ammonium chloride, ammonium sulfate and ammonium phosphate.
 6. The process according to claim 1, wherein the inhibitor is an organic reducing agent.
 7. The process according to claim 6, wherein the organic reducing agent is at least one member selected from the group consisting of urea and oxylic acid.
 8. The process according to claim 1, wherein said mandrel wire is made from molybdenum or nickel silver.
 9. The process according to claim 1, wherein said inhibitor is selected from the group consisting of potassium permanganate, potassium perchlorate, ammonium chloride, ammonium sulfate, ammonium phosphate, ammonium nitrate, urea and mixtures thereof.
 10. The process according to claim 9, wherein said inhibitor is a mixture of potassium permanganate and potassium perchlorate.
 11. The process according to claim 1 wherein said inhibitor is present in said resolving solution in an amount sufficient to absorb substantially all the gaseous nitrogen oxides produced when said filament coil is dipped into said resolving solution.
 12. The process according to claim 11, wherein said resolving solution contains at least about 3 percent inhibitor.
 13. The process according to claim 1, wherein said mandrel wire is a nickel silver alloy comprising about 45 to 65 percent copper, 60 to 35 percent nickel and 15 to 35 percent zinc. 